1. Field of the Invention
The present invention relates to an ethanol preparation method in which a biomass containing cellulose is used as a raw material.
2. Description of the Related Art
In recent years, from the viewpoint of preventing global warming, it has been called on to reduce emission amount of carbon dioxide which is considered to be one of the reasons of global warming. Therefore, there has been considered to use a mixed fuel of ethanol and a liquid hydrocarbon compound such as gasoline as a vehicle fuel. The ethanol can be yielded from fermentation of a plant material, for example, an agricultural crop such as sugar cane, corn or the like. Since the plant itself, the raw material of the plant material, has absorbed carbon dioxide via photosynthesis, even though the ethanol produced from the plant material is combusted, the emission amount of carbon dioxide is equal to the amount of carbon dioxide absorbed by the plant itself. In other words, the summed emission amount of carbon dioxide can be made theoretically equal to zero, which is the so-called carbon neutral effect. Therefore, the emission amount of carbon dioxide can be reduced in accordance with the amount of ethanol which is used to replace the liquid hydrocarbon compound such as gasoline.
However, if sugar cane, corn and the like are consumed in a large amount as the raw materials for preparing ethanol, there is a problem that the amount thereof supplied as food would be decreased.
In this regard, there has been considered a technology to produce ethanol by using an inedible biomass containing cellulose as a substituent to the plant material such as sugar cane, corn or the like. As examples of the biomass containing cellulose, wood, rice straw, haulm, bagasse, bamboo, pulp and waste materials originated therefrom, such as waste paper, may be given.
As a preparation method for ethanol, there has been known a method as disclosed, for example, in Japanese Patent Laid-open No. 2006-88136, in which the biomass containing cellulose is saccharifized by enzyme to yield a saccharide solution, thereafter, saccharide contained in the saccharide solution is fermented into ethanol by introducing ethanol fermentative bacteria into the saccharide solution. The saccharification of the cellulose by enzyme is performed by, for example, adding the biomass containing the cellulose, together with water and diastase, into a mush tun. The ethanol fermentation of the saccharide solution is performed by, for example, introducing the saccharide water solution yielded in the mush tun into a fermentor and introducing the ethanol fermentative bacteria into the saccharide water solution.
In the saccharification (glycosylation) of cellulose by enzyme, the enzyme activity decreases as the concentration of glucose yielded according to the saccharification increases. Thereby, in order to prevent the enzyme activity from decreasing, it is favorable to maintain the glucose concentration of the saccharide water solution in the mush tun equal to or less than 10 w %, more favorably, equal to or less than 5 w %.
On the other hand, in order to accelerate the ethanol fermentation of the glucose, it is desirable that the glucose concentration of the saccharide water solution is within a range of 15 to 20 w %. Therefore, it is necessary to condense preliminarily the saccharide water solution yielded according to the saccharification of cellulose by enzyme so that the glucose concentration is within the mentioned range prior to the ethanol fermentation of the saccharide water solution. Generally, distillation method is adopted to condense the saccharide water solution.
However, large amount of heat energy is needed to condense the saccharide water solution according to the distillation method, therefore, it is disadvantageous.